How to assign a hotkey to 'set origin' context menu?
Create a new key map item under 3D View > Object Non-modal and set the identifier to object.origin_set (bl_idname for Object.origin_set() operator).  In order to match the definition of the hotkey in 2.7x, set the event to C and enable ShiftCtrl Alt for the modifier keys:



Note: Assigning shortcuts to enum menus like this (operator in this case), is a known limitation and will hopefully supported in the near future: https://developer.blender.org/T65704